After I posted it, I thought "hmm I wonder if I should've elaborated", then thought "naaaah they'll get it". Clearly I should've elaborated. 
I reaaaallly get the impression that libertarians would split the republican party, and maybe take a little bit of the vote from democrats, but by and large not in any significant way. I don't know much about Weld, but Johnson was a republican if memory serves. Lots of republicans have talked about voting for the libertarian party instead. If you look at what libertarians stand for, like you said, they're largely a bunch of republicans who want to smoke pot. Sounds like a win-win to me!
Not to mention, if you get a third voice in the debates, my thinking is, there would be less time to spend on attacking each other and more time devoted to actual policy positions. When it comes down to it, I think we can all agree Clinton can win just about any debate she's in, as long as it's focused on issues and not mud-slinging. Which we all know Trump is great at, being the overgrown orangutan that he is.
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